Producing gas and oil wells necessarily include a borehole which extends downwardly from the surface of the earth to a subterranean zone containing hydrocarbons to be produced. A casing, which is a special steel tubing welded or screwed together, is lowered into the borehole to prevent collapse of the borehole. This casing is then generally cemented between the casing and the borehole wall for at least a portion of its length to further stabilize the borehole and provide additional strength against pressures which could collapse the borehole. Cementing the casing also provides the benefit of isolating zones which may produce oil, gas or water.
Production tubing is run axially within the casing. This tubing may hang free or be latched into a production packer. The space between the interior of the casing and the exterior of the tubing is commonly referred to as the tubing/casing annulus. To establish production the casing, surrounding cement sheath and the producing formation are perforated at desired depths by various methods as known to those skilled in the art.
In oil field operations it is often advantageous for a single well to produce from multiple zones which are by their nature higher and lower within the wellbore. A packer is set between these zones at a determined depth in order to isolate production from higher and lower perforated intervals (zones) within the wellbore. A production string, which traverses the upper zones and terminates in the packer, produces the lower zone. A second production string may be introduced into the same production tubing/casing annulus in order to produce the upper zones, or the upper zones may be produced without tubing, i.e., production is brought to the surface through the tubing/casing annulus.
When the reservoirs (zones) are producing, mobile granular material may enter the wellbore from the formation and this material may deposit within the annular space. In addition, precipitation of dissolved minerals and salts may occur and deposit material within the annular space as a result of temperature and pressure changes as fluids enter the wellbore.
At some point during the production life of a well it may be desirable to commingle the production from upper and lower zones. At this juncture it is critical that the location (depth) of fill and/or precipitates be known. In order to commingle production the long tubing string which traverses the upper zones is desirably perforated at a point above any granular fill or deposited precipitates.
Knowledge of the height of granular fill and/or deposited precipitates is important for determining:
1) whether perforations are covered by the fill material which may restrict production and necessitate a workover; PA1 2) a correct perforation point for the commingling of upper and lower zones separated by a packer; and PA1 3) a depth at which the tubing string is likely to be stuck.
It is further noted that tubing may become stuck because of granular fill, or the precipitation of minerals and salts in the wellbore, or because of the collapse of casing. To reduce the time that a workover rig is on location at the well site and the costs associated therewith, it is desirable to know the depth at which the tubing is stuck prior to the arrival of the rig on location.
Thus, there is presently a need, which would be available to the oil and gas producer, for accurate, rapid and economical measurement of the top of fill material accumulated in a tubing/casing annulus.
Accordingly, it is an objective of this invention to determine the location of the top of material that accumulates in the tubing/casing annulus of a well at a point of depth from the surface of the earth.
It is a more specific object of this invention to determine the top of fill material through the use of gamma radiation measurements.
Another object is to determine the top of fill location in a rapid, economical and accurate manner.
Still another object is to operate a well in an efficient manner.